3. Nuclear energy and hydropower each account for about one-sixth of the world=
s electricity generation, however.

III. Nuclear Power

1. Nuclear power is free of air pollution, but its promise has been clouded by nuclear weaponry, the dilemma of radioactive waste disposal, and the long shadow of Chernobyl and other power plant accidents.

A. Fission releases nuclear energy.

1. Nuclear energy is the energy that holds protons and neutrons together within the nucleus of an atom.

2. We convert this energy into thermal energy, which can then be used to generate electricity.

3. The reaction that drives the release of nuclear energy in power plants is nuclear fission, the splitting apart of atomic nuclei.

B. Nuclear energy comes from processed and enriched uranium.

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1. Nuclear reactors are the facilities within nuclear power plants where we control fission and generate electricity.

2. Uranium ore is in finite supply, which is why nuclear power is generally considered a nonrenewable energy source.

3. Uranium is used for nuclear power because it is radioactive. Radioactive isotopes, or radioisotopes, emit subatomic particles and high-energy radiation as they decay into lighter radioisotopes, until they become stable isotopes.

4. Each radioisotopes decays at a rate determined by that isotope=
s half-life, the amount of time it takes for one-half of the atoms to give off radiation and decay.

5. After several years in a reactor, enough uranium has decayed so that the fuel loses its ability to generate adequate energy, and it must be replaced. Some countries reprocess the spent fuel to recover more usable energy. Most of the fuel, however, is disposed of as radioactive waste.

C. Fission in reactors generates electricity in nuclear power plants.

D. Breeder reactors make better use of fuel, but have raised safely concerns.

1. Breeder nuclear fission makes use of 238U, which in conventional fission goes unused as a waste product.

2. Because 99% of all uranium is 238U, breeder fission makes much better use of fuel, generates far more power, and produces far less waste than conventional fission.

3. However, breeder fission is considerably more dangerous than conventional nuclear fission. This is because highly reactive liquid sodium, rather than water, must be used as a coolant, raising the risk of explosive accidents.

4. Breeder fission can potentially be used to supply plutonium to nuclear weapons programs.

5. Because of these reasons, nearly all of the world=
s breeder reactors have now been shut down.

E. Fusion remains a dream.

1. Nuclear fusion - the process responsible for the immense amount of energy that our sun generates and the force behind hydrogen or thermonuclear bombs - involves forcing together the small nuclei of lightweight elements under extremely high temperature and pressure.

2. The hydrogen isotopes deuterium and tritium can be fused together to create helium, releasing a neutron and a tremendous amount of energy.

3. Overcoming the mutually repulsive forces of protons in a controlled manner is difficult, and fusion requires temperatures of many millions of degrees Celsius. Despite decades of research, fusion experiments still require more energy to be input than is produced by the process.

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4. Successful fusion in a reactor would produce vast amounts of energy using water as a fuel, with only low-level radioactive wastes and no risk of accidents, sabotage, or weapons proliferation.

F. Nuclear power delivers energy more clearly than fossil fuels.

1. Researchers calculate that nuclear power lowers emissions 4-150 times below those of fossil fuel combustion.

2. Uranium generates far more power than coal by weight or volume, requiring less of it to be mined, causing less damage to landscapes, and generating less solid waste.

3. A drawback to nuclear power is the radioactive waste that must be disposed of in a way that minimizes danger to prevent and future generations.

4. A second main drawback is that if an accident occurs at a power plant, or if a plant is sabotaged, the consequences can be potentially catastrophic.

5. Most governments, although not necessarily most citizens, have judged the good to outweigh the bad, and today there are 439 operating nuclear plants around the world.

G. Nuclear power poses small risks of large accidents.

1. Two events were influential in shaping public opinion about nuclear energy. The first was at a nuclear plant in Pennsylvania.

a. In 1979, the Three Mile Island plant in Pennsylvania experienced the most serious nuclear power plant accident in the United States.

b. Through a combination of mechanical failure and human error, metal surrounding the uranium fuel rods began to melt, releasing radiation.

c. This process, a meltdown, proceeded through half of one reactor core at Three Mile Island.

d. The accident was brought under control within days, the damaged reactor was shut down, and multi-billion-dollar cleanup efforts stretched on for years.

e. Although no significant health effects to residents have been proven in the years since, the event put safety concerns squarely on the map for U.S. citizens and policymakers.

H. Chernobyl saw the worst accident yet.

1. In 1996, and explosion at the Chernobyl plant in Ukraine caused the most severe nuclear power plant accident the world has yet seen.

2. Human error, combined with unsafe reactor design, resulted in explosions that destroyed the reactor and sent clouds of radioactive debris billowing into the atmosphere.

3. There is now a gigantic concrete sarcophagus around the demolished reactor, but the landscape for at least 30 km around the plant remains contaminated.

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4. Atmospheric currents carried radioactive fallout from Chernobyl across much of the Northern Hemisphere.

I. Waste disposal remains a problem.

1. Even if nuclear power generation can be made completely sage, we still must dispose of the spent fuel rods and other material and equipment in a location where radiation will not escape to harm the public.

2. This waste remains dangerous for thousands of years.

3. Currently, nuclear waste from power generation is being held in temporary storage at nuclear power plants around the world.

4. In the United States, Yucca Mountain, Nevada, has been chosen for nuclear waste storage beginning in 2010.

I. Multiple dilemmas have slowed nuclear power=
s growth.

1. Almost every nuclear plant has turned out to be more expensive than expected.

2. Plants have aged more quickly than expected because of problems that were underestimated. Well over 100 plants around the world have been shut down, having served on average for less than half their expected lifetimes.

3. Shutting down a plant can be more expensive than the original construction.

4. These issues make nuclear-power-generated electricity more expensive than electricity from other sources.

5. Asian nations are adding generating capacity, but in Western Europe there are no reactors under construction today and 75% of the current capacity will be retired by 2030.

6. Nearly half of the U.S. nuclear plants ordered since 1957 have been cancelled.

IV. Biomass Energy

1. Biomass consists of the organic material that makes up living organisms. Biomass energy involves burning many types of plant and animal matter, including wood, charcoal, and combustible animal waste products.

A. Fuelwood and other traditional biomass sources are widely used in the developing world.

1. In hydroelectric power, or hydropower, the kinetic energy of moving water is used to turn turbines and generate electricity.

A. Modern hydropower uses dams and A
run-of-river@
approaches.

1. Most of our hydroelectric power today comes from impounding water in reservoirs behind dams and then letting that water pass through the dam. Because immense amounts of water are stored behind dams, this is called the storage technique.

2. An alternative is the run-of-river approach, in which electricity is generated without greatly disrupting the flow of river water. This approach sacrifices the reliability of water flow across seasons, but minimizes many of the environmental impacts.

B. Hydroelectric power is widely used.

1. For nations with large amounts of river water and the economic resources to build dams, hydroelectric power has been a keystone of their development and wealth.

C. Hydropower is clean and renewable.

1. Hydropower is renewable as long as rain still falls and fills rivers and reservoirs.

2. Hydropower is cleaner than power from fossil fuels.

3. Recent evidence indicates that large reservoirs may release the greenhouse gas methane as a result of anaerobic decay.

D. Hydropower has negative environmental impacts.

1. Damming rivers destroys habitat for riverside wildlife as riparian areas above dam sites are submerged and those below dam sites often become starved of water.

2. Because water discharge is carefully controlled to optimize electricity generation, the natural flooding cycles of rivers are disrupted.

4. Dams also cause thermal pollution, because water downstream from dams may become unusually warm if water levels are kept unnaturally shallow, and then may experience periodic flushes of cold water from the lower depths of the reservoir.

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5. Dams generally block the passage of fish and other aquatic creatures, effectively fragmenting segments of the river and reducing biodiversity in each segment.

E. Hydropower may not expand much more.

1. Most of the world=
s rivers that offer excellent opportunities for hydropower have been dammed.

2. Moreover, in developed nations, awareness of the environmental impacts of dams is causing people to propose dismantling and removing of the dams that exist to restore river habitats.

3. In the United States, 98% of the rivers appropriate for dam construction are already dammed, and many of the remaining 2% are protected from dam building.

4. Hydropower will likely continue to increase in developing nations, but in developed nations growth will likely slow or stop.

VI. Conclusion

A. Many nations have sought to diversify their energy portfolios with alternative energy sources.

B. The three most developed and widely used alternatives so far are nuclear power, biomass energy, and hydropower.